I have used FlowXpress plenty of times with no problems. I use it for a basic idea of how my Intake manifolds (performance cars) will act under pressure.

The latest one I have built with the sheet metal features to be more accurate when I actually go to cut/form the parts. This is the first time I do this. Normally I would just build solid parts and shell them or parts that look like sheet metal but not use the actual sheet metal functions. Doing it this way I have never had problems. Now no matter what I do, it still finds openings. But I have no idea where these openings are. Zooming in and tracing the edges, I can not find any gaps. I even made small parts to mate between where the sheet metal parts join each other that interfere with both parts to ensure there are no gaps.

Is there another way to test for gaps? Can FlowXpress not be used with sheet metal parts?

Flow Simulation has a tool for finding gaps and cracks, but I do not believe this tool is available to Express users. If your model is simple I would just rebuild it as a shelled solid, with the way SolidWorks handles sheet metal folding it may be impossible to find every crack.

Unfortunately I cannot think of a simple way to do this. What I have done in the past is to seal up the cracks with a long tube, something approximating a weld bead. But you said you already tried that so unfortunately I do not have any other advice for you.